(1) Field of the Invention
The present invention relates to picture coding that corresponds to motion prediction field coding of an interlaced-picture signal.
(2) Description of the Related Art
In recent years, two methods called progressive video and interlaced video have been used as an uncompressed picture data format. In the progressive video, all of the sample lines for a frame are composed of sample lines with the same time. On the other hand, in the interlaced video, the sample lines for a frame are composed of sample lines with alternatingly different times. A picture signal in the interlaced video is provided in a way that one frame consists of a top field and a bottom field compiled on sample line basis.
Since the above uncompressed picture data is a large data volume, it is general that the data is compressed and recorded by using a coding method such as MPEG2 or H.264/MPEG4-AVC. In the coding method such as MPEG2 or H.264/MPEG4-AVC, a motion vector between frames having different time is detected and an inter picture coding process through motion compensation prediction is used. Also, it has two modes: frame coding that codes through inter picture motion prediction by each frame; and field coding that codes through inter picture motion prediction by each field.
Furthermore, in the H.264/MPEG4-AVC method, it is possible to select a reference frame from a plurality of frames and compensate its motion. For example, in a case of the field coding, it is possible to select which field, a top field or a bottom field, is referred from fields previously coded.
When a reference field is selected like this, it is possible to select one with higher coding efficiency and better picture quality as after motions of all possible fields are detected as a candidate and coded. However, there is an issue that this method requires a large amount of processing volume.
As a conventional example to solve a problem like this, there are Patent Reference 1 (see Japanese Unexamined Patent Application Publication No. 2006-94454 referred to as Patent Reference 1 hereinafter) and Patent Reference 2 (see Japanese Unexamined Patent Application Publication No. 2007-259206 referred to as Patent Reference 2 hereinafter). In the Patent Reference 1 and the Patent Reference 2, a moving picture coding apparatus that selects a reference field based on a size of motions moving between fields (a motion vector in an entire image through a compressed picture, an average value of the motion vector, an average value of absolute values of the motion vector, and so on) is suggested. Also, in Patent Reference 3 (see Japanese Unexamined Patent Application Publication No. 2008-11117 referred to as Patent Reference 3 hereinafter), it discloses a reference picture decision method that selects a reference field based on a correlation between fields in addition to the size of motions between fields. Furthermore, in Patent Reference 4 (see International Publication No. 2007/040197 referred to as Patent Reference 4 hereinafter), as a decision criteria for a reference field selection, it discloses a reference field selection method that presumes a size of motions from a motion vector based on an compressed picture, a ratio of an intra-coded macroblock, and so on, and that selects a reference field based the size of the presumed motion.
In the conventional reference field selection method, a reference field is selected by using a size of motions predicted by each field (by each picture) as a decision criterion. Because of this, there is an issue that, even if a background of a picture to be coded remains static and some object partially moves substantially, it is decided that there is a movement as an average motion by each field so that a reference field is undesirably selected based on the concerned decision result.
In addition, when an object in a lower portion of an image moves substantially on a static background, coding efficiency and picture quality of a part having a movement on the lower portion of the image are optimized but coding efficiency and picture quality of a static area on an upper portion of the image are downgraded. There is an issue that deteriorated picture quality becomes remarkable in the entire image because it is easier to detect a distortion in the static area than the moving area.